Rosenthal in 30's found that in a welding using electric arc, the "hill of heat" moves on a weld center line and brings about melting of metals and as the molten metal solidifies weld results. He stated the hill of heat is rigid in physical dimensions and moves over center line of two stationary plates placed in juxta position for butt welding and therefore welding is a quasi stationary process.
I found that in every welding operation, "a cup of molten metal runs at the welding speed on the centerline of two plates placed in juxta position for butt welding and as the cup of molten metal runs at a welding speed, a dynamically stable state is created at the trailing edge of the weld (TEW). The resultant cup in solid metals is also rigid in physical dimensions, as long as welding parameters and thickness of plate on which it is created are constant. In the next state, the molten metal in the cup enters into solidifying state. Thus a Second Quasi Stationary State (SQS) state is created in welding operation. This SQS state has two components: the Liquid Metal Zone (LMZ) and Solidifying Metal Zone (SMZ). LMZ in the cup is under the arc; the SMZ follows the LMZ and adjoins it, in a gradual manner, during solidification process. Due to heat flux directions in the LMZ, a specific depth of penetration and shape of the cup results in the solid metals of the base plates. This function utilizes the superheat input to the weld through arc.
At the beginning of the SMZ which follows LMZ, easy and free growth of a crystal from the solid-liquid interface is obstructed by growth of numerous crystals in its vicinity having numerous directions of growths. Therefore, a complex micro structure results in unassisted weld solidification process. The numerous directions of growths of crystals is due to numerous cooling gradients that come into effect instantly because of numerous directions of exposed micro structures on the solid-liquid interface, in the solidifying metal. The curved surface demarcated in solid metal, seen in cross section of a solidified weld, when cross section is made at right angles to the direction of welding, also assists in bringing about complexity in the directional growth of micro structure. With obstructions from numerous crystals growing around a crystal or grain, the resultant micro structure becomes dense with several entanglements of crystals or with voids which may capture gas as temperature falls.
As the heat is dissipated, the crystal/grain structure contracts and results in stress generation at individual entanglements depending upon the property of metals involved. Therefore total stress we call residual stress is generated in a weld. This is the primary stress that causes secondary reaction stresses to come into effect in welded base plates. Thus, we see distortion in the welded component.
This can be compared to a simple but compact structure due to mass of crystals and grains that can grow in a resultant one direction, by my process described below.
If we consider total heat quantity input to a weld as A, it normally contents three parts, B, C and D. Part B is superheat responsible for bringing about the penetration and the specific shape of cup in the solid base metals. And C is the quantity of heat that is dissipated in the base metals. D is the heat radiation into atmosphere in a welding operation. Quantity D is neglected as it is usually constant. Solid sites of micro structure in the surface of the cup are exposed to the molten metal. A polycrystalline solid material such as steel, has micro structure with faces exposed in several directions at the interface. These form sites for crystals to grow in solidifying metal in the SMZ. There is excess heat quantity C in the volume of cup at solidification temperature after formation of the cup in solid base metals. This excess heat is conducted in various directions in base metals. Because a crystal must grow naturally in the direction opposite to cooling gradient established, a overall complex crystal structure is established in the solidifying molten metal. In the process developed, some part of quantity C of the heat is conducted into the material deposited on the TEW. In the experiment described to illustrate the CTEW process, the heat extraction melts the material deposited on TEW because of the deposited material has melting point lower than the temperature of the solidified surface at TEW and the semi-solid weld metal beneath it. Thus, heat quantity "c" is extracted in TEW on which heat sink is created in preference to its dissipation into base metals. Thus, CTEW process controls heat dissipation into base metals and develops cooling gradients in the SMZ. This promotes overall unidirectional growth of mass of crystals in the SMZ. Such unidirectional growth of crystals drives out gas molecules. Thus porosity is eliminated. Since entanglements of crystals are reduced. Thus, the stress generated at such entanglements as the mass of crystals solidifies is also reduced. Thus, incidence of micro-cracks is also reduced.
The CTEW process is different from the Sub Merged Arc Process (SAW) as in SAW process, it is the arc which is submerged with different objectives. In the CTEW process it is the TEW that is covered, objective being to modify the micro structure.
In unassisted welding solidification process, stresses generated at micro structure level, are known to be characterized with short span and high values. Such stresses can be relieved only by post weld heat treatment in absence of application of processes of this inventor covered under U.S. Pat. Nos. 4,386,727 and 5,024,371 or application of the present CTEW process. During incubation period in a post weld heat treatment, such as post heat treatment in a furnace, such stress is known to be relieved. One mechanism known is micro plasticity. Neither the incubation time necessary nor the precise temperature at which such a post weld treatment operation should be carried out is ever determined. Further it is obvious that such post weld heat treatments may reduce the stresses in distorted part but may or may not relieve the primary stress entirely in the weld that caused distortion. In fact relaxation of primary stress that caused distortion is alone necessary. Further such post weld heat treatments may not be applicable to parts with large size, as facilities of heat treatment can be cost prohibitive. Such postweld treatments may not remove porosity.